The Sphere of the World with Commentaries

an astrolabe in the clarity of a starry night, with the pole sighted through both holes of the alidade, the multitude of degrees in which the alidade stands is noted; then, let the cosmometer proceed directly towards the north from the south until, in the clarity of another night, with the pole sighted as before, the alidade stands one degree higher. After this, let the space of this journey be measured, and 700 stadia will be found. Then, with 700 stadia given to each of the 360 degrees, the circumference of the terrestrial globe will be found. From these, however, according to the rule of the circle and diameter, the diameter of the Earth will be able to be found in this way: take away the twenty-second part from the circuit of the Earth, and the third part of the remainder—that is 80,181 and a half and a third of one stadium—will be the diameter or thickness of the terrestrial globe.

CHAPTER TWO

ON THE CIRCLES FROM WHICH THE MATERIAL SPHERE IS COMPOSED, AND THAT SUPRACELESTIAL SPHERE WHICH IS UNDERSTOOD TO BE COMPOSED THROUGH THIS ONE.

Of these circles, some are greater, some smaller, as is evident to the senses. For a greater circle in a sphere is said to be that which, described on the surface of the sphere over its center, divides the sphere into two equal parts. A lesser one, however, is that which, described on the surface of the sphere, does not divide it into two equal parts, but into unequal portions. Among the greater circles, one must first speak of the equinoctial. The equinoctial, therefore, is a circle dividing the sphere into two equal parts according to any part of itself, equidistant from each pole. And it is called the equinoctial because when the sun passes through it—which is twice in the year, namely at the beginning of Aries and at the beginning of Libra—there is an equinox in the whole Earth. Whence it is also called the equator of day and night, because it equalizes the artificial day to the night. And it is called the girdle of the first motion. Whence it is to be known that the first motion